Abstract: High Resolution Seismic/Sequence Stratigraphy of Intra-Slope Basins, Western Gulf of Mexico

Beaubouef, R. T. and S. J. Friedmann - Exxon Production Research Co.; B. W. Alwin - Exxon Exploration Co.

High resolution 2D seismic, sidescan sonar images, and shallow penetration cores were used to study a portion (approximately 40-mile length) of the upper to middle Texas continental slope. Within the study area are four intra-slope basins presently connected to one another via a network of submarine channels. The depositional setting occurs within 1,500-5,000 feet of present day water depth and is located depositionally down-dip of Pleistocene fluvio-deltaic complexes considered to represent the source of sediment delivered to this portion of the slope. Three of these basins are filled, while the fourth, most southerly basin is presently underfilled. Based on seismic stratigraphy, and facies the fill of these basins is interpreted to exhibit vertical cyclicity reflecting alternating deposition of mass transport complexes (MTC), distributary channel-lobe complexes (DLC), leveed-channel complexes (LCC), and hemipelagic drape complexes (DC). MTC's are characterized by erosional bases, irregular tops and low amplitude, chaotic seismic facies and are interpreted as mud-rich complexes of slumps, slides, and debris flow deposits. The MTC's are typically overlain by seismic facies units characterized by moderate to high reflection amplitude and continuity interpreted as DLC's. Representing the most sand- rich portions of basin fill, the DLC's exhibit onlapping, compensatory and locally erosional internal reflection geometries and fan shaped or distributary map patterns. These complexes are interpreted to have been deposited from highly concentrated sediment gravity flows delivered across the basin floor through a network of relatively small distributary channels and contain both channelized and sheet-like depositional elements. LCC's form a distinct seismic facies characterized by low amplitude, highly continuous reflection character and "gull -wing" cross-sectional profile. LCC's are considered to form from overbank deposition of low concentration turbidites and contain low to moderate sand percentages. DC's are thin, high amplitude continuous seismic units that represent hemipelagic mudstones deposited during periods of abandonment and sediment starvation.

A multi-stage depositional model is presented to explain a) the vertical and lateral arrangement of these units within sequences, b) the vertical and lateral arrangement of sequences within sequence sets, c) changes in depositional style during basin filling, and d) the progressive downslope filling of basins. Within this framework, alternating deposition of MTC's and DLC's dominate the early stages of basin filling. LCC's are not observed to have formed during early cycles of deposition, and are interpreted as elements formed during the latter stages of basin filling and represent an integral part of the sediment transport system linking the basins. However, post-depositional processes have significantly modified these channels and the fill of the basins. Most notable among these are headward erosion and mass wasting due to upslope migration of nickpoints in response to local gradient changes along the system as individual basins were filled and subsequently bypassed by sediment gravity flows.

AAPG Search and Discovery Article #90933©1998 ABGP/AAPG International Conference and Exhibition, Rio de Janeiro, Brazil